U.S. patent number 10,522,327 [Application Number 16/044,770] was granted by the patent office on 2019-12-31 for method of operating a charged particle beam specimen inspection system.
This patent grant is currently assigned to Applied Materials Israel Ltd.. The grantee listed for this patent is Applied Materials Israel Ltd.. Invention is credited to Michal Avinun-Kalish, Gilad Erel, Stefan Lanio.
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United States Patent |
10,522,327 |
Erel , et al. |
December 31, 2019 |
Method of operating a charged particle beam specimen inspection
system
Abstract
A charged particle beam specimen inspection system is described.
The system includes an emitter for emitting at least one charged
particle beam, a specimen support table configured for supporting
the specimen, an objective lens for focusing the at least one
charged particle beam, a charge control electrode provided between
the objective lens and the specimen support table, wherein the
charge control electrode has at least one aperture opening for the
at least one charged particle beam, and a flood gun configured to
emit further charged particles for charging of the specimen,
wherein the charge control electrode has a flood gun aperture
opening.
Inventors: |
Erel; Gilad (Mazkeret Batia,
IL), Avinun-Kalish; Michal (Nes-Ziona, IL),
Lanio; Stefan (Erding, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Applied Materials Israel Ltd. |
Rehovot |
N/A |
IL |
|
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Assignee: |
Applied Materials Israel Ltd.
(Rehovot, IL)
|
Family
ID: |
55180758 |
Appl.
No.: |
16/044,770 |
Filed: |
July 25, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180330919 A1 |
Nov 15, 2018 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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14446146 |
Jul 29, 2014 |
10056228 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J
37/28 (20130101); H01J 37/026 (20130101); H01J
2237/0048 (20130101); H01J 2237/0044 (20130101) |
Current International
Class: |
H01J
37/02 (20060101); H01J 37/28 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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102005358 |
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Apr 2011 |
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CN |
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2000-208579 |
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Jul 2000 |
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JP |
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2008-165990 |
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Jul 2008 |
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JP |
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201342420 |
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Oct 2013 |
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TW |
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2011/058950 |
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Apr 2013 |
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WO |
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Other References
Search Report for Taiwan Invention Patent Application No.
104124406, dated Nov. 28, 2018, 1 page. cited by applicant .
U.S. Appl. No. 14/446,146 Restriction Requirement dated Oct. 21,
2016, 7 pages. cited by applicant .
U.S. Appl. No. 14/446,146 Non-Final Office Action dated Mar. 30,
2017, 14 pages. cited by applicant .
U.S. Appl. No. 14/446,146 Final Office Action dated Aug. 24, 2017,
14 pages. cited by applicant .
U.S. Appl. No. 14/446,146 Notice of Allowance dated May 3, 2018, 7
pages. cited by applicant.
|
Primary Examiner: Ippolito; Nicole M
Assistant Examiner: Luck; Sean M
Attorney, Agent or Firm: Kilpatrick Townsend & Stockton
LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is division of Ser. No. 14/446,146, filed Jul. 29,
2014, the entire contents of which are incorporated herein by
reference in its entirety for all purposes.
Claims
The invention claimed is:
1. A method of operating a charged particle beam specimen imaging
system, comprising: biasing a charge control electrode to a first
potential; moving a specimen support table for positioning a first
portion of a specimen below a flood gun aperture opening in the
charge control electrode; pre-charging the first portion of the
specimen with charged particles emitted from a flood gun; and
moving the specimen support table for positioning the first portion
of the specimen below a first aperture opening in the charge
control electrode, wherein the first aperture opening is aligned
with an optical axis of an objective lens of a scanning charged
particle beam unit, wherein a path of a charge particle beam
extends through the objective lens and the first aperture opening,
and a path of the charged particles emitted from the flood gun
extends through the flood gun aperture opening without passing
through the objective lens.
2. The method according to claim 1, further comprising: scanning
the charged particle beam focused by the objective lens over an
area of the first portion of the specimen.
3. The method according to claim 2, wherein scanning the charged
particle beam is conducted while the charge control electrode is
biased to the first potential.
4. The method according to claim 2, further comprising: biasing the
charge control electrode to a second potential different from the
first potential before scanning the charged particle beam.
5. The method according to claim 1, further comprising: biasing the
specimen to an operation voltage applied while scanning the charged
particle beam; moving the specimen support table for positioning a
calibration target below the flood gun aperture opening in the
charge control electrode; and measuring a current of charged
particles emitted from the flood gun while the calibration target
is biased to the operation voltage.
6. The method according to claim 5, further comprising: moving the
specimen support table while measuring the current of the flood gun
for characterizing a profile of the charged particles emitted from
the flood gun.
7. The method according to claim 2, further comprising:
pre-charging or dis-charging a second portion of the specimen while
scanning the charged particle beam focused by the objective lens
over the area of the first portion of the specimen.
Description
TECHNICAL FIELD OF THE INVENTION
Embodiments of the present invention relate to devices for imaging
a specimen, e.g. a wafer, with one or more charged particle beams
and including a flood gun. Embodiments of the present invention
particularly relate to a charged particle beam specimen inspection
system having an objective lens and a flood gun, specifically to a
charged particle beam specimen inspection system, a multi-beam
specimen inspection system, and a method of operating a charged
particle beam specimen inspection system.
BACKGROUND OF THE INVENTION
Charged particle beam apparatuses have many functions, in a
plurality of industrial fields, including, but not limited to,
electron beam (wafer) inspection, critical dimensioning of
semiconductor devices during manufacturing, defect review of
semiconductor devices during manufacturing, exposure systems for
lithography, detecting devices and testing systems. Thus, there is
a high demand for structuring, testing and inspecting specimens
within the micrometer and nanometer scale.
Micrometer and nanometer scale process control, inspection or
structuring is often done with charged particle beams, e.g.
electron beams, which are generated and focused in charged particle
beam devices, such as electron microscopes or electron beam pattern
generators. Charged particle beams offer superior spatial
resolution compared to, e.g. photon beams due to their short
wavelengths.
Particularly for electron beam inspection (EBI) technology,
throughput is of foremost interest. It is inter alia referred to,
in particular, to surface inspection at low landing energies
<500 eV and low secondary electron (SE) extraction fields.
Normally, for high current density electron probe generation,
compound objective lenses are used (superimposed magnetic and
electrostatic retarding field lenses). In those lenses, the
electron energy inside the column is reduced to the final landing
energy. Further, for the purpose of pre-charging a wafer to a
desirable surface potential, for example in order to increase
detection sensitivity of voltage contrast (VC) defects in the wafer
fabrication process, or to dis-charge/neutralize wafer charging
effects, a flood gun can be used.
In view of the above, it is beneficial to provide an improved
charged particle beam device and a method of operating thereof that
overcome at least some of the problems in the art.
SUMMARY OF THE INVENTION
In light of the above, an improved charged particle beam wafer
inspection system, an improved multi-beam wafer imaging system, and
an improved method of operating a charged particle beam wafer
imaging system according to the independent claims are provided.
Further advantages, features, aspects and details are evident from
the dependent claims, the description and the drawings.
According to one embodiment, a charged particle beam specimen
inspection system is provided. The system includes an emitter for
emitting at least one charged particle beam, a specimen support
table configured for supporting the specimen, an objective lens for
focusing the at least one charged particle beam, a charge control
electrode provided between the objective lens and the specimen
support table, wherein the charge control electrode has at least
one aperture opening for the at least one charged particle beam,
and a flood gun configured to emit further charged particles for
charging of the specimen, wherein the charge control electrode has
a flood gun aperture opening.
According to another embodiment, a multi-beam specimen inspection
system is provided. The multi-beam specimen inspection system
includes a charged particle beam specimen inspection system. The
charged particle beam specimen inspection system includes an
emitter for emitting at least one charged particle beam, a specimen
support table configured for supporting the specimen, an objective
lens for focusing the at least one charged particle beam, a charge
control electrode provided between the objective lens and the
specimen support table, wherein the charge control electrode has at
least one aperture opening for the at least one charged particle
beam, and a flood gun configured to emit further charged particles
for charging of the specimen, wherein the charge control electrode
has a flood gun aperture opening. The multi-beam specimen
inspection system further includes at least one further emitter for
emitting at least one further charged particle beam, wherein the
charge control electrode has at least one further aperture opening
for the at least one further charged particle beam.
According to yet another embodiment, a method of operating a
charged particle beam specimen imaging system is provided. The
method includes biasing a charge control electrode to a first
potential, moving a specimen support table for positioning a first
portion of a specimen below a flood gun aperture opening in the
charge control electrode, pre-charging the first portion of the
specimen with charged particles emitted from a flood gun, and
moving the specimen support table for positioning the first portion
of the specimen below a first aperture opening in the charge
control electrode, wherein the first aperture opening is aligned
with an optical axis of an objective lens of a scanning charged
particle beam unit.
BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the above recited features of the
present invention can be understood in detail, a more particular
description of the invention, briefly summarized above, may be had
by reference to embodiments. The accompanying drawings relate to
embodiments of the invention and are described in the
following:
FIG. 1 illustrates a schematic partial view of a scanning charged
particle beam device with a flood gun according to embodiments
described herein;
FIG. 2 illustrates a schematic view of a scanning charged particle
beam device with a flood gun according to embodiments described
herein;
FIG. 3A illustrates a schematic view of a charge control electrode
for a charged particle beam wafer inspection system according to
embodiments described herein;
FIG. 3B illustrates a schematic view of a conductive mesh, which
may be provided to close an aperture opening in a charge control
electrode according to embodiments described herein;
FIG. 4 shows a schematic of a flood gun, which is provided in a
charged particle beam inspection system according to embodiments
described herein;
FIGS. 5A and 5B illustrate schematic views of a calibration target,
which can be utilized in a charged particle beam wafer inspection
system according to embodiments described herein;
FIG. 6 shows a schematic view of a multi-beam wafer imaging system
having a flood gun according to embodiments described herein;
and
FIG. 7 shows a flow chart of a method of operating a charged
particle beam inspection system, according to embodiments described
herein;
DETAILED DESCRIPTION OF EMBODIMENTS
Reference will now be made in detail to the various embodiments of
the invention, one or more examples of which are illustrated in the
figures. Within the following description of the drawings, the same
reference numbers refer to same components. Generally, only the
differences with respect to individual embodiments are described.
Each example is provided by way of explanation of the invention and
is not meant as a limitation of the invention. Further, features
illustrated or described as part of one embodiment can be used on
or in conjunction with other embodiments to yield yet a further
embodiment. It is intended that the description includes such
modifications and variations.
Without limiting the scope of protection of the present
application, in the following the charged particle beam device or
components thereof will exemplarily be referred to as an electron
beam device including the detection of secondary electrons and/or
backscattered electrons, which are also referred to as signal
electrons. Embodiments can still be applied for apparatuses,
systems and methods, in which the charged particle beam may
alternatively be an ion beam. Embodiments can still be applied for
apparatuses and components detecting corpuscles such as secondary
and/or backscattered charged particles in the form of electrons or
ions, photons, X-rays or other signals in order to obtain a
specimen image. Generally, when referring to corpuscles they are to
be understood as a light signal in which the corpuscles are photons
as well as particles, in which the corpuscles are ions, atoms,
electrons or other particles.
A "specimen" or "wafer" as referred to herein, includes, but is not
limited to, semiconductor wafers, semiconductor workpieces, and
other workpieces such as memory disks, masks, substrates for flat
panel displays and the like. According to some embodiments, a
specimen can be selected from the group consisting of: a wafer, a
mask, a substrate for a flat panel display, and a flat panel
display. Embodiments of the invention may be applied to any
workpiece which is structured or on which material is deposited. A
specimen or wafer includes a surface to be imaged and/or structured
or on which layers are deposited, an edge, and typically a
bevel.
According to some embodiments, which can be combined with other
embodiments described herein, the apparatus and methods are
configured for or are applied for electron beam inspection (EBI),
critical dimension measurement and defect review applications,
where the microscopes and methods according to embodiments
described herein, can be beneficially used in light of high
throughput of these applications. According to some embodiments
described herein, an E-beam inspection (EBI), critical dimension
measurement (CD) tool, and/or defect review (DR) tool can be
provided, wherein high resolution, large field of view, and high
scanning speed can be achieved. According to embodiments described
herein, a wafer imaging system or a wafer SEM inspection tool
refers to EBI tools, CD tools or DR tools, which are specific tools
as understood by a person skilled in the art.
In the context of the here described embodiments, without limiting
the scope of protection thereto, an intermediate beam acceleration
system intends to describe a charged particle beam apparatus with
initial high acceleration of the charged particles which will be
decelerated to a landing energy shortly before striking the
specimen or wafer. The energy or velocity ratio
v.sub.acc/v.sub.landing between the acceleration velocity v.sub.acc
at which the charged particles are guided through the column and
the landing velocity v.sub.landing at which the charged particles
strike the specimen can be about at least 10 or higher, e.g. 20 or
higher. Furthermore, the landing energy can be 2 keV or less, e.g.
1 keV or less, such as 500 eV or even 100 eV.
Embodiments described herein relate to systems being a single or
multi column scanning electron microscope having a flood gun. The
flood gun is provided such that the objective lens and the flood
gun shares a charge control electrode and/or the flood gun is
provided to be at least partially within the objective lens
housing. The scanning electron microscope and the flood gun are
combined together in one wafer inspection apparatus. According to
some embodiments, which can be combined with other embodiments
described herein, the objective lens and the flood gun share at
least some electrostatic components and/or are provided in a common
magnetic environment. Accordingly, the throughput of a wafer
inspection system can be further improved.
According to some embodiments, the flood gun is configured to
generate a high emission current with a large spot size. The high
emission current and the large spot size enable scanning and
charging of large surfaces to a desired potential in a short time.
According to some embodiments, which can be combined with other
embodiments described herein, the emission current of the flood gun
can be up to 5 mA, for example, 50 .mu.A to 500 .mu.A, such as 100
.mu.A to 300 .mu.A. According to yet further additional or
alternative embodiments, the spot size in the plane of the
specimen, e.g. a wafer, can be 7 mm or below, for example 3 mm to 6
mm, such as about 5 mm. The beam energy of the flood gun can,
according to some examples, be 300 to 3000 eV.
Combining a flood gun in a scanning electron beam inspection system
according to embodiments described herein can be beneficial in
light of one or more of the following aspects. (1) The flood gun
and the scanning electron beam inspection system can share one or
more of the optical elements, for example the charge control
electrode above the wafer. Accordingly, further power supplies and
respective controllers may be shared. This can inter alia reduce
the costs of ownership and/or the system complexity. (2) The
provision, additionally or alternatively, of a common magnetic
environment by an objective lens housing can shield magnetic fields
of nearby components, for example nearby SEM columns. (3) The
common charge control electrode allows for charging the specimen or
wafer surface utilizing the flood gun with the same charging
conditions as compared to the charging conditions of a column of
the electron beam inspection system. (4) The need for an alignment
between the charging system and the scanning system can be reduced.
(5) The stage movement between the charging system and the scanning
system can be reduced. Accordingly, the time for stage movement
and/or navigation errors can be reduced. (6) The specimen or wafer
can be held at the same bias below the scanning system and the
flood gun, wherein the cycle time between charging and scanning is
reduced. Accordingly, it may even be possible to pre-charge one
portion on a wafer while scanning or inspecting another portion of
the wafer. The above aspects allow for increased throughput and/or
reduced cost of ownership.
According to embodiments described herein, the objective lens for
an electron beam system, i.e. the last lens before the electron
beam impinges on the specimen or wafer, includes a
magnetic-electrostatic lens. As shown in FIG. 1, the electrostatic
lens component includes an upper electrode 162, which lies on a
high potential and a lower electrode, e.g. charge control electrode
166, which lies on a potential close to the sample voltage and
which decelerates the electrons for providing the desired landing
energy. These electrodes contribute to focusing the beam as well as
to slowing the beam down to the desired low primary beam
voltage.
FIG. 1 shows a portion of a scanning electron microscope 100. The
objective lens includes the magnetic lens assembly 60 having an
upper pole piece 63, a lower pole piece 64 and a coil (not shown in
FIG. 1). The objective lens further includes an electrostatic lens
component having a first electrode 162, i.e. upper electrode in the
figures, and a charge control electrode 166, i.e. lower electrode
in the figures. Further, a control electrode 170 for control of the
signal electrons or the extraction field acting on the signal
electrons respectively is provided at a position along the optical
axis 2 from the position of the charge control electrode 166 to the
specimen support table 50 or the specimen 52 respectively. In FIG.
1, the control electrode 170 is provided within the charge control
electrode 166. The control electrode 170 can, for example, have
essentially the same position along the optical axis as the charge
control electrode 166. The charge control electrode 166 can also be
referred to as big proxi or large proxi and the control electrode
170 can also be referred to as small proxi. According to some
embodiments, the small proxi can be at the same distance from the
specimen as the large proxi. According to other embodiments, the
small proxi is closer to the specimen as the large proxi.
According to the embodiments described herein, it is understood
that the small proxi, i.e. the control electrode 170, has a small
influence on the properties of the electrostatic lens component,
yet is sufficiently small enough to be considered an individual
element, with the functionality to control the extraction of the
SEs from the specimen or the guidance of SEs released from the
specimen.
The objective lens 60 focuses the electron beam 12, which travels
in the column along optical axis 2, on the specimen 52, i.e. in a
specimen plane. The specimen 52 is supported on a specimen support
table 50. According to some embodiments, which can be combined with
other embodiments described herein, scanning of an area of the
specimen can be conducted by movement of the table in a first
direction essentially perpendicular to the optical axis and by
scanning lines in another, second direction essentially
perpendicular to the optical axis and essentially perpendicular to
the first direction.
A flood gun 152 is provided in the scanning electron microscope
100. As shown in FIG. 1, the flood gun 152 shares the charge
control electrode 166 with the scanning inspection system of the
scanning electron microscope. According to embodiments described
herein, the charge control electrode 166 has at least one aperture
opening 162 and a flood gun aperture opening 154. The specimen
support table can be moved to a first position, in which the
electron beam 12 impinges on the specimen 52, for example a
position as shown in FIG. 1. The specimen support table 50 can
further be moved to a second position, in which charged particles
emitted from the flood gun impinge on the specimen 52, for example
a wafer.
According to yet further embodiments, which can be combined with
other embodiments described herein, a charged particle beam wafer
inspection system, such as the scanning electron microscope 100
shown in FIG. 1, includes an objective lens housing 65. The
objective lens housing 65 surrounds the objective lens, and
particularly the upper pole piece 63 and the lower pole piece 64.
For example, the objective lens housing 65 is magnetically
insulated from the pole pieces by an air gap or a magnetic
insulator, i.e. a material with a relative permeability
.mu./.mu..sub.0=1, such as copper or the like. According to some
embodiments, which can be combined with other embodiments described
herein, the objective lens housing 65 can include a material having
a relative permeability .mu./.mu..sub.0 of 10000 or above, for
example mu-metal or the like. The flood gun 152 and the objective
lens 60 share the same magnetic environment by having the objective
lens housing 65 surrounding at least a portion of the flood gun
152. The objective lens housing can shield the fields of nearby SEM
columns or other devices for the objective lens 60 and the flood
gun 152.
According to some embodiments described herein, the objective lens,
can be electrostatic, magnetic or combined magnetic-electrostatic.
A magnetic lens or a magnetic lens assembly can be provided by a
permanent magnet, a coil, or a combination thereof. For example a
the objective lens can have a magnetic lens assembly including one
or more pole pieces. According to embodiments described herein, an
objective lens having surrounds the objective lens and shields one
or both of magnetic fields and electrostatic fields. The objective
lens housing surrounds at least a portion of the flood gun.
Accordingly, the flood gun can be placed close to the objective
lens.
Sharing at least one of the charge control electrode 162 and the
objective lens housing 65 allows for reduced costs and a small
footprint of the inspection system. Further, the flood gun 152 can
be provided at a distance from the scanning electron beam
components such that the specimen support table 50 can move the
specimen 52 from the electron beam to a position below the flood
gun 152 and vice versa in a reduced time.
Further embodiments can be described with respect to FIG. 2. FIG. 2
shows a charged particle beam device, such as an SEM imaging
apparatus, i.e. scanning electron microscope 100 having a flood gun
152. The electron beam column 20 provides a first chamber 21, a
second chamber 22 and a third chamber 23. The first chamber, which
can also be referred to as a gun chamber, includes the electron
source 30 having an emitter 31 and suppressor.
According to embodiments described herein, the emitter 31 is
connected to a power supply for providing a voltage to the emitter.
The emitter can be an emitter of one or more emitters of an emitter
assembly. For the examples described herein, the potential provided
to the emitter is such that the electron beam is accelerated to an
energy of 8 keV or above. Accordingly, typically the emitter is
biased to a potential of -8 keV or higher negative voltages, e.g.
in the case where the column and the beam guiding tube, which also
provides the first electrode 162 in FIG. 2, are on ground
potential. However, higher beam energies inside the column, e.g. 20
keV or higher, will be even more advantageous for the electron
optical performance (e.g. resolution or current density). As
described above, having the emitter on a positive potential is a
typical embodiment with the benefits that the column and the beam
guiding tube can be at ground or at a moderate potential. Yet, with
respect to the focusing properties of the zoom lens according to
embodiments described herein, the emitter could also be grounded
and a power supply could be connected to the electrode 162 shown in
FIG. 2.
An electron beam is generated by the electron beam source 30. The
beam is aligned to the beam-shaping aperture 450, which is
dimensioned to shape the beam, i.e. blocks a portion of the beam.
Thereafter, the beam passes through the beam separator 380, which
separates the primary electron beam and the signal electron beam,
i.e. the signal electrons. The primary electron beam is focused on
the specimen 52 or wafer by the objective lens. The specimen is
positioned on the specimen stage, i.e. a specimen support table 50.
On impingement of the electron beam, for example, secondary or
backscattered electrons are released from the specimen 52, which
can be detected by the detector 398. Even though backscattered
electrons and secondary electrons are typically detected by the
detector, some passages of this disclosure relate to secondary
electrons only, i.e. as a comparison to primary electrons, and it
is understood that backscattered electrons are also considered to
be signal electrons or similar to secondary electrons as understood
herein, i.e. there are secondary products for signal generation of
the image.
According to some embodiments, which can be combined with other
embodiments described herein, a condenser lens 420 and a beam
shaping or beam-limiting aperture 450 is provided. The two-stage
deflection system 440 is provided between the condenser lens and
the beam-shaping aperture 450 for alignment of the beam to the beam
shaping aperture. According to embodiments described herein, which
can be combined with other embodiments described herein, the
electrons are accelerated to the voltage in the column by an
extractor or by the anode. For example, the extractor can be
provided by the first (upper) electrode of the condenser lens 420
or by a further electrode (not shown). According to yet further
embodiments, the condenser lens may also be a magnetic condenser
lens for controlling the probe diameter.
Further, a scanning deflector assembly 370 is provided. For
example, the scanning deflector assembly 370 can be a magnetic, but
preferably an electrostatic scanning deflector assembly, which is
configured for high pixel rates. According to typical embodiments,
which can be combined with other embodiments described herein, the
scanning deflector assembly 370 can be a single stage assembly as
shown in FIG. 2. Alternatively, also a two-stage or even a
three-stage deflector assembly can be provided. Each stage of the
deflector assembly can be provided at a different position along
the optical axis 2.
Signal electrons, e.g. secondary and/or backscattered electrons,
are extracted from the wafer or specimen e.g. by a control
electrode and are further accelerated within the objective lens.
The beam separator 380 separates the primary electrons and the
signal electrons. The beam separator can be a Wien filter and/or
can be at least one magnetic deflector, such that the signal
electrons are deflected away from the optical axis 2. The signal
electrons are then guided by a beam bender 392, e.g. a
hemispherical beam bender, and a lens 394 to the detector 398.
Further elements like a filter 396 can be provided. According to
yet further modifications, the detector can be a segmented detector
configured for detecting signal electrons depending on the starting
angle at the specimen.
An objective lens housing 65 surrounds the objective lens 60.
Further, at least a portion of the flood gun 152 can be surrounded
by the objective lens housing 65. According to some embodiments,
the objective lens and the flood gun can have a common objective
lens housing. The charge control electrode 166 is provided between
the wafer or specimen 52 (or the wafer support table 50,
respectively) and the common objective lens housing 65. This allows
for controlling the charging potential of the specimen surface,
e.g. the wafer surface, under the flood gun and the scanning
electron microscope column. The voltage difference between the
wafer and the charge control electrode determines the resultant
wafer potential. Flood gun electrons, i.e. the charged particles
emitted from the flood gun, pass the charge control electrode 166
through the flood gun aperture opening 154. According to some
embodiments, which can be combined with other embodiments described
herein, the flood gun aperture opening can be covered with a mesh
254 or grid. The mesh or grid can improve the uniformity of the
electrostatic field above the specimen. Accordingly, the uniformity
of the charging profile can be improved with the mesh 254, e.g. a
grid.
FIG. 2 shows a power supply 261 for the flood gun 152. The power
supply 261 is provided in an electrical cabinet 260. Further, a
power supply 262 is provided, wherein the charge control electrode
166 and the wafer or the specimens support table 50, respectively,
can be biased to the desired potentials. Further, the controller
263 for controlling the movement of the specimen support table 50
can be provided. According to embodiments described herein, power
supplies and individual controllers can be controlled by a main
controller 250, such as a main computer having at least a CPU and a
memory.
According to yet further embodiments, which can be combined with
other embodiments described herein, a calibration target 240 can be
provided on the specimen support table 50. Details of the
calibration target 240 are described with respect to FIGS. 5A and
5B. The calibration target 240 is connected with the power supply
262 for biasing the specimen 52 or the specimen support table 50,
respectively.
The calibration target is configured for characterizing the beam of
electrons emitted from the flood gun 152 and/or for measuring the
current emitted from the flood. The emission current of the flood
gun can be up to 5 mA, for example, 50 .mu.A to 500 .mu.A, such as
100 .mu.A to 300 .mu.A. The high emission current of the flood gun
allows for a better throughput of the inspection system since
pre-charging and/or dis-charging can be conducted in a much shorter
time. According to yet further additional or alternative
embodiments, the spot size in the plane of the specimen or wafer
can be 7 mm or below, for example 3 mm to 6 mm, such as about 5 mm.
Accordingly, the current density is lower when using a flood gun,
for example in the range of 1 to 10 .mu.A/mm.sup.2. This reduces
the likelihood of having artifacts when inspecting a specimen, for
example a wafer. Yet, the higher emission current allows for
charging some types of layers to the desired potential, which could
not be charged with the electron beam of the scanning electron beam
column. Particularly, layers having a large capacitance may not be
charged to the desired potential with an electron beam of a
scanning electron microscope.
Embodiments described herein can be utilized for or can include
pre-charging a wafer to a desirable surface potential, for example
in order to increase detection sensitivity of voltage contrast (VC)
defects in the wafer fabrication process, and scanning an electron
beam of a scanning electron beam microscope over the pre-charged
surface thereafter. According to some embodiments, the uniformity
of pre-charging over a scanned area can be 10 V peak-to-peak or
below. For example, the specimen, such as a wafer, can be charged
to 100 V.+-.5V.
As shown in FIG. 2, the flood gun 152 uses some common interface
with the SEM column. The charge control electrode 166, which is
configured to be used during pre-charging, i.e. operation of the
flood gun, and which is configured to be used during inspection
with a scanning electron beam, is provided in the chamber 23. That
is, the flood gun and the SEM column are operated under the same
vacuum condition, i.e. they share the same pressure within the
vacuum chamber.
The specimen support table 50 includes an X-Y-stage navigation
system, which is configured to move the specimen, for example a
wafer, under the SEM column and/or the flood gun. The wafer is
biased to a voltage potential, which determines the landing energy
of the electrons on the wafer.
FIG. 3 shows a charge control electrode 166. The charge control
electrode 166 has an opening 154. Charged particles, for example
electrons, which are emitted from the flood gun can pass through
the opening 154 of the charge control electrode 166. The charge
control electrode further includes openings 162. The example shown
in FIG. 3A shows five openings 162 for five electron beams of a
scanning electron beam system. The charge control electrode is also
provided in the chamber 23, i.e. the vacuum chamber, as shown in
FIG. 2, and is common for the SEM column and the flood gun.
Accordingly, the charge control of the flood gun and the SEM column
is controlled by the same high voltage controller.
A conductive mesh 254 is provided at the flood aperture opening or
within the flood gun aperture opening. That is, the aperture
opening is covered with a thin metal mesh or a grid in order to
generate a uniform and planar electrostatic field between the
specimen, for example a wafer, and the charge control electrode.
The conductive mesh 254 can be biased to the potential of the
charge control electrode. By biasing the conductive mesh, a uniform
and planar electrostatic field can be provided. This improves the
uniformity of the profile of the pre-charging.
As shown in FIG. 3B, the conductive mesh 254 includes a plurality
of wires 354. A first plurality of wires 354 extend in a first
direction and a second plurality of wires 354 extend in a second
direction, which is different from the first direction. For
example, the second direction can be essentially perpendicular to
the first direction. For example, the second direction can have an
angle of 80.degree. to 100.degree. with respect to the first
direction. The first plurality of wires 354 and the second
plurality of wires 354 form the mesh 254.
According to some embodiments, which can be combined with other
embodiments described herein, the first direction of the first
plurality of wires and the second direction of the second plurality
of wires is not parallel to one of the specimen movement directions
of the specimen support table 50, which may for example move in an
X-direction and a Y-direction. Further, additionally or
alternatively, the first direction of the first plurality of wires
and the second direction of the second plurality of wires are not
parallel to one of the scanning directions of the charged particles
emitted from the flood gun, which can be deflected by a beam
deflection system within the flood gun. The scanning directions may
also correspond to the X-director and the Y-direction of the
specimen support table 50. Particularly, the first direction of the
first plurality of wires and the second direction of the second
plurality of wires can have an angle of 30.degree. to 60.degree.,
for example about 45.degree., with respect to the X-direction or
the Y-direction. Providing such an angle can avoid an uncharged
line on the surface of the specimen when scanning the flood gun
electrode over the specimen.
The conductive mesh 254 can have one or more protrusions 355. The
protrusions 355 can be used to provide a fixed orientation of the
first direction of the first plurality of wires and the second
direction of the second plurality of wires with respect to the
specimen movement direction and/or scanning direction. The
protrusions 355 may further be utilized for an electrical
connection between the conductive mesh and the charge control
electrode. Yet further, the protrusions may serve for easy
replacement of the conductive mesh. A typical mesh may include
wires having a thickness of 5 .mu.m to 100 .mu.m. A typical mesh
may be manufactured to have spaces between the wires of 80 .mu.m to
200 .mu.m. The ratio between the dimension of the wires and the
dimension of this basis determines a blocking ratio, which may be
10% to 30%, for example about 20%.
FIG. 4 shows the flood gun 152. The flood gun has a housing 402.
Electrons are emitted from the emitter 404 and accelerated by the
anode 405. According to some embodiments, a beam blanker system 406
can be provided. The beam blanker system can deflect the beam of
electrons, such that the electrons are blocked by the beam blocker
407. An electrode 408 can be provided in order to focus the beam of
charged particles. Accordingly, some embodiments may include a
focusing option. The first beam scanning system 412 for deflecting
the beam in a first direction, for example an X-direction, and the
second beam scanning system 414 for deflecting the beam in a second
direction, for example a Y-direction, can be provided. The beam
deflection system of the flood gun is configured to align the flood
gun electrons to pass through the center of the aperture opening in
the charge control electrode. The aperture-opening diameter is
limiting the flood gun beam size on the wafer plane to the desired
size and shape. The diameter limitation can further serve to avoid
charging in undesired regions.
FIG. 4 further shows the charge control electrode 166 having an
aperture opening provided therein. The aperture opening in the
charge control electrode 166 is closed by the conductive mesh 254.
The electrons emitted from the flood gun 152 impinge on a specimen
52, for example a wafer. According to embodiments described herein,
the emission current can be controlled by controlling the source
voltage. By controlling the source voltage, a constant emission
current can be provided. The focusing lens provided by the
electrode 408, for example provided by the potential of the
electrode in combination with other electrodes and potentials
within the flood gun 152, allows for controlling the spot diameter
of the electrons emitted from the flood gun. Control of the spot
diameter is beneficial in order to avoid flooding outside of the
wafer surface during charging or the calibration target surface
during calibration. Flooding outside of the desired region, for
example on cables or the like may deteriorate the operation of the
charged particle beam inspection system due to the high beam
currents emitted by the flood gun. The control of the flooding on
the desired services can further be improved by the beam blanker
406 and/or the first beam scanning system and the second beam
scanning system, which may deflect the beam of electrons emitted
from the flood gun.
FIG. 4 further shows the working distance 430, i.e. the distance of
the lower portion of the housing 402 from the surface of the
specimen. The housing 402 of the flood gun does not only provide a
compartment for the components provided therein but also defines
the region in which the potentials within the flood gun influence
the electron beam of the flood gun. Accordingly, the working
distance 430 is provided between the lower portion of the housing
402 and the surface of the specimen 52. According to some
embodiments, which can be combined with other embodiments described
herein, the working distance can be from 60 mm to 90 mm, for
example from 70 mm to 80 mm. The combination of beneficial choice
of the working distance with the conductive mesh allows for the
desired uniformity of the electrostatic field between the wafer and
the charge control electrode. This allows the charging profile to
be within a desired range of 10 V peak-to-peak or below.
FIG. 5A shows the calibration target 240. The calibration target
240 is for example positioned on the specimen support table 50 as
shown in FIG. 2. The calibration target 240 can be utilized for
characterizing the beam of the flood gun 152 and/or for measuring
the current emitted from the flood gun 152. In order to calibrate
the flood gun, the specimen support table 50 is moved such that the
electrons from the flood gun impinge on the calibration target 240.
Accordingly, the calibration target is located on the stage
assembly. The calibration target allows for controlling the voltage
potential of the specimen support table. According to embodiments
described herein, the beam emitted from the flood gun can be
characterized with the specimen support table, or the specimen
respectively, is biased to the operating voltage.
As shown in FIG. 5A, the calibration target 240 is connected to the
power supply 262 for biasing the wafer or the specimen support
table, respectively. Variations in the power supply signal while
scanning the flood gun spot over the calibration target can be
analyzed. This analysis allows characterization of the flood gun
spot with the power supply signals of the power supply 262.
Contrary to the Faraday cup, which is typically grounded for
measuring the current, the calibration target according to some
embodiments described herein operates under a biased condition.
The calibration target 240 has an aperture plate 540 with at least
one opening. The electron beam emitted from the flood gun can pass
through the opening in the aperture plate 540. The electron beam
impinges on a cup 542. The cup 542 includes an electron absorbing
material, e.g. conductive material. A current detection device 562
in the power supply 262 provides the signal indicative of the
current in the power supply. During flood gun calibration the
current in the power supply 262 is zero if the electron beam of the
flood gun is switched off. If the electron beam of the flood gun is
switched on, the current can be detected in the power supply.
According to some embodiments, which can be combined with other
embodiments described herein, the calibration target 240 is biased
during the flood gun calibration. For example, the calibration
target is biased to the same potential as the wafer curing imaging
and/or pre-charging of the wafer with the charged particle beam
inspection system. Since the target is biased to the potential of
the wafer and/or the specimen support table, there is a low risk of
high voltage damages.
According to some embodiments, which can be combined with other
embodiments described herein, the aperture plate 540 can include at
least two openings. As shown in FIG. 5B the aperture plate 540 can
include a small opening 551 and a large opening 552. The small
opening 551 has a diameter, which is smaller than the diameter of
the electron beam emitted from the flood gun. For example, the
diameter of the small opening 551 can be 1 mm or below. The large
opening 552 has a diameter which is larger than the diameter of the
electron beam emitted from the flood gun. For example, the diameter
of the large opening 552 can be 5 mm or above.
For the measurement of the total current, the calibration target
240 is positioned, for example by moving the specimen support
table, such that the electron beam emitted from the flood gun
passes through the large opening 552 of the aperture plate 540. The
entire current is collected in the cup 542 and the resulting
current is measured in the voltage supply 262. For characterizing
the beam emitted from the flood gun, the calibration target 240 is
positioned, for example by moving the specimen support table, such
that the electron beam emitted from the flood gun partly passes
through the small opening 551 of the aperture plate 540. The
position of the small opening 551 can be varied by scanning the
specimen support table, for example in X and Y directions. Since
the small opening 551 is smaller than the beam diameter, only a
portion of the electron beam emitted from the flood gun passes
through the aperture plate and is collected by the cup 542.
Measuring the current detected in the voltage supply 262 as a
function of the position of the specimen support table, i.e. the
position of the small opening 551, allows for generating a current
profile of the electron beam. Accordingly, the electron beam
emitted from the flood gun can be characterized. For example, the
shape of the electron beam can be measured with the calibration
target 240.
FIG. 6 illustrates yet further embodiments, wherein a retarding
field scanning microscope, i.e. wafer imaging system 400 is
provided as a multi-beam device. Typically, two or more beams can
be provided in a multi-beam device. As an example, FIG. 5 shows
five emitters 5 such that five electron beams are emitted in the
gun chamber 520. This corresponds to the five aperture openings in
the charge control electrode shown in FIG. 3A. The emitter tips are
biased to an acceleration potential V.sub.acc by voltage supply 4,
which provides a potential to the tips as compared to ground 3.
Electrodes 512, e.g. extractors or anodes can be provided, e.g. in
a cup-like shape. These electrodes are electrically insulated by
insulators 532 with respect to each other and with respect to the
gun chamber 520. According to some embodiments, which can be
combined with other embodiments described herein, also two or more
of the electrodes selected from the group consisting of extractor
and anode can be provided. Typically, these electrodes 512 are
biased to potentials by voltage supplies (not shown) in order to
control the two or more electron beams.
The charged particle beams travel in a further chamber 530, in
which a specimen 52 is provided. The objective lens 560 focuses the
beams on the specimen or in a specimen plane, respectively. The
objective lens can have a magnetic lens assembly 60 with a common
magnetic lens portion, i.e. a magnetic lens portion acting on two
or more of the charged particle beams. For example, one common
excitation coil is provided to a pole piece unit or a pole piece
assembly, wherein several openings for passing of the two or more
electron beams through the pole piece unit are provided. The one
common excitation coil excites the pole piece unit, such that, for
example, one beam is focused per opening. Power supply 9 can
provide the current for the magnetic lens portion of the objective
lens.
As shown in FIG. 6, the objective lens 560 further includes an
electrostatic lens component 360. For example, an electrostatic
lens portion 560 having one or more first electrodes and a second
electrode are provided. The second electrode can be a charge
control electrode with aperture openings for the scanned electron
beams and an aperture opening for charged particles emitted from
the flood gun. 152. As shown in FIG. 6, the first electrode can
also be provided as a separated electrode for one or more of the
electrostatic lens portions. That is the first electrode can be
separate and/or independent of a beam guiding tube in the column.
This can also apply for the single beam columns described herein.
Further, for each of the electrons beams, a control electrode can
be provided.
Three power supplies 462, 466 and 470 are shown in FIG. 6. Some of
the power supplies have exemplarily five connection lines for
respective electrodes for each of the five electrostatic lens
components. For example, power supply 462 can be connected to the
respective first electrodes, power supply 466 can have a single
connection to the common charge control electrode, and power supply
470 can be individually connected to the respective control
electrodes. The controller 460 is connected to the voltage supplies
462, 466 and 470 for the electrodes of the electrostatic lens
components and the control electrodes. The various connection lines
entering the column housing from some of the power supplies (the
rest of which is omitted for better overview) illustrate that each
of the electrodes for the individual beams can be controlled
independently. However, it can be understood that one or more of
the electrodes of the electrostatic lens components and one or more
of the control electrodes can also be biased with a common power
supply. Further, it is noted that particularly the power supply 462
can be omitted if the first electrode is grounded as explained
above.
According to some embodiments, the objective lens can be provided
according to any of the embodiments described herein. It has to be
considered that particularly for EBI applications, but also for
CD/DR applications, as compared to common wafer imaging, throughput
is a critical aspect to be considered. The operational modes
described herein are useful for high throughput. Also cold field
emitters (CFE) and thermally assisted field emitters (TFEs) can be
used to increase the throughput. Accordingly, the combination of a
flood gun according to embodiments described herein with a CFE, a
thermally assisted field emitter, or a Schottky emitter is
particularly useful. As a further implementation, a combination
with a multi-electron beam device as e.g. described with respect to
FIG. 6 further provides a specific combination, which can be
considered beneficial for the throughput of wafer inspection.
According to different embodiments, which can be combined with
other embodiments described herein, a multi-beam wafer inspection
system can include two or more beams, wherein one beam each can be
provided in two or more columns, wherein two or more beams can be
provided in one column, or both, i.e. two or more columns can be
provided, wherein each of the two or more columns include two or
more beams on the specimen, e.g. a wafer. If two or more columns
are provided, they may share some components, e.g. the charge
control electrode. If two or more beams are provided in one column
they may be generated by a combination of a multi-opening aperture
plate and a deflection system such that two or more virtual sources
are generated.
The embodiments described herein, may as well include additional
components (not shown) such as condenser lenses, deflectors of the
electrostatic, magnetic or compound electrostatic-magnetic type,
such as Wien filters, stigmators of the electrostatic, magnetic or
compound electrostatic-magnetic type, further lenses of the
electrostatic, magnetic or compound electrostatic-magnetic type,
and/or other optical components for influencing and/or correcting
the beam of primary and/or signal charged particles, such as
deflectors or apertures. Indeed, for illustration purposes, some of
those components are shown in the figures described herein. It is
to be understood that one or more of such components can also be
applied in embodiments of the invention.
According to some embodiments, a method of operating a charged
particle wafer imaging system is provided. A flow chart of the
method of operating a charged particle wafer imaging system is
shown in FIG. 7. The method includes biasing a charge control
electrode to a first potential as shown in box 702. A specimen
support table is moved for positioning a first portion of a wafer
below a flood gun aperture opening in the charge control electrode
(see reference numeral 704). As indicated by box 706 the first
portion of the wafer is pre-charged with charged particles emitted
from a flood gun. After pre-charging the specimen support table is
moved (see reference numeral 708) for positioning the first portion
of the wafer below a first aperture opening in the charge control
electrode, wherein the first aperture opening is aligned with an
optical axis of an objective lens of a scanning charged particle
beam unit.
By pre-charging the first portion of the wafer with a flood gun
before imaging the first portion with a scanning charged particle
beam unit, the pre-charging can be conducted faster as compared to,
for example, pre-charging with the scanning charged particle beam
unit itself. Accordingly, throughput can be increased. Further, the
first portion of the wafer is provided below the charge control
electrode while being moved from the pre-charging position to the
imaging position. This is beneficial for improved charge control on
the wafer to be inspected.
According to some embodiments, which can be combined with other
embodiments described herein, the specimen or wafer (or the
specimen support table, respectively) can also be biased to a
specimen potential, for example a high potential, during imaging of
the first portion of the wafer. Yet further, the specimen or wafer
can be biased to the same specimen potential during pre-charging of
the first portion of the wafer. Accordingly, the potential
difference, i.e. the voltage, between the wafer and the charge
control electrode is not varied when moving from the pre-charging
position to the imaging position.
According to yet further embodiments, which can be combined with
other embodiments described herein, the specimen support table can
be moved to a position such that the calibration target, which can
be provided on the specimen support table, is moved to a position
below the flood gun. The charged particles, for example the
electrons, emitted by the flood gun may then impinge on the
calibration target. Impingement of electrons of the flood gun on
the calibration target allows for measuring the current of the
flood gun and/or characterizing the electron beam emitted from the
flood gun. According to some embodiments described herein, the
measurement of the current and/or the characterization of the
electron beam of the flood gun can be conducted while the
calibration target is biased to the potential, for example a high
potential, such as the potential provided to the wafer or specimen
support table during pre-charging and/or imaging of the wafer. For
the measurement of the current and/or the characterization of the
electron beam of the flood gun, the current in the power supply for
biasing the wafer or the specimen support table can be measured
upon impingement of electrons from the flood gun.
According to alternative embodiments, either the total current
emitted from the flood gun can be measured with the calibration
target or the profile of the emission current can be measured for
characterizing of the electron beam emitted from the flood gun. For
measurement of the total current, the calibration target can
include the large aperture opening in an aperture plate, which is
larger than the beam diameter of the electron beam emitted from the
flood gun. The entire beam of the flood gun can enter the
calibration target for measurement of the current. For measurement
of an admission profile of the electron beam emitted from the flood
gun, the calibration target can include a small aperture opening in
the aperture plate, which is smaller than the beam diameter of the
electron beam emitted from the flood gun. For example, the small
diameter can be 2 mm or below, for example 1 mm or below, such as
about 0.5 mm. In light of the aperture being smaller than the beam
diameter, only a portion of the electron beam emitted from the
flood gun passes through the aperture opening. By scanning the
small aperture opening relative to the electron beam emitted from
the flood gun, by scanning the electron beam emitted from the flood
gun relative to the small aperture opening, or by scanning both the
electron beam emitted from the flood gun and the small aperture
opening of the calibration target, the current profile of the
electron beam emitted from the flood gun can be measured.
According to yet further embodiments, which can be combined with
other embodiments described herein, the flood gun may also be
provided for dis-charging of the portion of the wafer. For example,
after positioning the first portion of the wafer below the first
aperture opening in the charge control electrode and imaging the
first portion of the wafer or at least area of the first portion of
the wafer, charge may build up on the area of the wafer upon
imaging of the area of the wafer. For dis-charging the area of the
first portion of the wafer, the specimen support table may be moved
back for positioning the first portion of the wafer below the flood
gun aperture opening. The area of the first portion of the wafer
can be dis-charged with the flood gun. The specimen support table
can be moved back to the imaging position. In the imaging position,
the imaging of the first portion of the wafer can be continued.
Embodiments described herein refer to an imaging charged particle
beam unit, wherein the focused charged particle beam is scanned
over the specimen, in combination with the flood gun, wherein the
flood gun and the imaging charged particle beam unit, for example
electron beam column, share the charge control electrode such as a
proxi electrode. Further, the flood gun and the imaging charged
particle beam unit can share a power supply for biasing the charge
control electrode. According to yet further additional or
alternative implementations, the flood gun and the imaging charged
particle beam unit can share an objective lens housing, such that a
common magnetic environment is provided for the flood gun and the
imaging charged particle beam unit. Based upon the sharing of the
charge control electrode and/or the objective lens housing, a
beneficial distance between the flood gun and the imaging charged
particle beam unit, for example a scanning electron microscope, can
be provided. This beneficial distance allows for sufficient
separation between the flood gun and the imaging charged particle
beam unit. Yet, the flood gun and the imaging charged particle beam
unit are close enough to allow for increased throughput, for
example based upon reduced movement time of the specimen support
table. Beyond that, according to some embodiments, the distance may
further allow for pre-charging or dis-charging of one portion of a
wafer while another portion of the wafer is imaged with the imaging
charged particle beam unit. Accordingly, throughput can be further
improved.
According to yet further details of some embodiments, which can be
combined with other embodiments described herein, the flood gun
charged particle source can be heated to different temperatures
during operation of the charged particle beam wafer inspection
system. For example, the flood gun charged particle source, such as
the flood gun electron source, can be heated to an operation
temperature for emitting charged particles, such as electrons.
While the flood gun is not used for emitting charged particles,
such as electrons, the flood gun charged particle source can be
heated to a second temperature lower than the operation
temperature. The second temperature can be a temperature
sufficiently low enough such that no electrons are emitted from the
charged particle source of the flood gun. Lowering the temperature
during and idle time of the flood gun enables increasing the
lifetime of the flood gun charged particle source.
While the foregoing is directed to embodiments of the invention,
other and further embodiments of the invention may be devised
without departing from the basic scope thereof, and the scope
thereof is determined by the claims that follow.
* * * * *